Using an LED to write or draw onto paper?

Last night i went to a bar in brixton after seeing massive attack. In the bar there were frames on the walls with white paper inside and we were given LED torches similar to these LED Torch. When the torch is on you can use it to write or draw onto the frame and it leaves a glowing trail which slowly fades. I really want to know how its done, as it was very entertaining and quite trippy and i want to make one myself. If anyone has any ideas please reply.

Cheers

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westfw10 years ago
Sounds like "standard" glow-in-the-dark phosphors; you can get this in the form of glow-in-the-dark paint, vinyl sheeting, or bare powder that you can mix or mould yourself. UV, Blue, and white LEDs are especially good at charging this stuff up.

The "new" phosphors based on strontium Aluminate are much brighter and longer lasting than the stuff from 10+ years ago (based on Zinc sulfide.) It does tend to be pretty expensive. Extreme Glow used to have some "seconds" of vinyl sheet that was a very good value, but it looks like they've sold out. Glow Inc is another company with a good reputation and a lot of info on their web site.
VIRON westfw10 years ago
glow-in-the-dark-plastic-sheet with pen lights. I agree, I've played with that.
LasVegas10 years ago
Sounds more like a grease pen display. Simply a sheet of Plexiglas with a cold cathode or florescent lamp on the edge. When the plexiglas is written on with various colored grease pens the grease glows.
GRL did something like this, but on a larger scale. They used laser pointers onto a building some how.
that was just a big projection unit hooked up to a pointer.
schorhr A.C.E.10 years ago
Another idea would be using LEDs (or prebuild dot matrix displays) behind a milky glass sheet or so.
LEDs will respond a bit like solar cells when light falls on them, so theoretically you can use them to switch on when light falls on them.

 _____________/  ________|       |               |V LED   - 100nF<>100uF  (V)-       -               ||_______|_______________|

If you close the switch you can mesure a short current at (V).

I have seen a circuit using this to charge and blink A LED in bright sunlight.
I am sure, using a flipflop or microcontroller system, you could switch a LED if a laserpointer goes across it.
jacknaz (author)  Weissensteinburg10 years ago
Thanks for the quick response but it wasnt anything to do with projection. It must me the paper that was inside the frame, maybe light sensitive. Also the colour of the drawing depends on the colour of the LED light. Hmm
westfw10 years ago
You could also throw massive amounts of computing power at it and come up with something like MultiTouch Computer Display video
well there could be some kind of light behind the paper (IR?)